add reducer estimater based on input size

This patch is for estimating the reducer number based on the input size. This idear is borrowed from pig and hive which also do the similar thing.  The default setting is allocate 1 reducer for 1gb input data. User can change it by setting parameter "cascading.exec.reducers.bytes.per.reducer" 

BTW, is there any contribution guideline document for cascading ? Because my code style may be not compatible with the official code base.